An aircraft gas turbine engine typically employs a number of sensors for the purpose of optimally controlling the engine according to the various state variables detected by the sensors, and the outlet pressure (P3) of the compressor is one of the important control parameters or state variables, particularly in conducting a fuel control using Qfr (fuel flow rate)/P3 (compressor outlet pressure) as a parameter at the time of accelerating or decelerating the engine. By using this parameter, it becomes possible to maintain a favorable air/fuel ratio and avoid misfires. For this reason, it is highly important to obtain a correct value of the compressor outlet pressure P3 at all times.
Should a sensor such as a compressor outlet pressure sensor fail, the resulting abnormal control parameter value would prevent a satisfactory operation of the engine. Therefore, it is highly essential to prepare a countermeasure for the time of a sensor failure.
One known precautionary measure is to use a plurality of sensors for a single control parameter, and detect any fault by comparing the output values of the different sensors. However, on account of the structure of the engine and manufacturing cost, it is difficult to provide a plurality of sensors for monitoring the outlet pressure of the compressor.
Because of such a restriction, it has been previously proposed to filter the output signal when the sensor for the compressor outlet pressure indicates an abnormal output value and to substitute the abnormal sensor output value with a fixed value. It is also known to switch to a non-load operating mode when the sensor output is detected to be abnormal. See Japanese patent laid open publication No. 2003-184578.
However, these known measures are known to have some problems. For instance, depending on the operating condition of the engine, the difference between the substitute value and actual value may be so great that an excessive time may be required for the engine to decelerate, and much depends on the discretion of the engine operator in operating the engine in a stable manner. Also, the control process disclosed in the mentioned patent publication is effective for use in a stationary engine, but is practically unusable for an aircraft engine.